Close coupled gas atomization of liquid metal is being developed as a process for forming metal powders. In close coupled gas atomization, a cylindrical gas plenum having an inner chamber in communication with an annular orifice, or an annular array of discrete orifices produce a jet, which may be comprised of an array of jets, that converge below a melt guide tube extending axially through the gas plenum. A stream of liquid metal passing through the melt guide tube and exiting therefrom is atomized in the annular jet of atomizing gas converging in the stream.
When the gas atomization of liquid metal is commenced, there is an opportunity to view the atomization of the liquid metal from viewports in the atomization chamber. In the atomization process, the atomizing gas flows at supersonic speeds resulting in great scattering and recirculation of the particulate formed by the atomization process. Soon after the atomization starts producing powdered material, recirculating powder from the atomization process obscures the view. In fact, the observation of the atomization nozzle is obscured within seconds after the process is started.
Information regarding the nature of the interaction between the atomizing gas and the liquid metal at the atomization nozzle can be obtained immediately at the start of the atomization process and before the viewing path to the atomization nozzle is obscured by the recirculating powder produced by the atomization process. However, it has not been possible to view the atomization process for more than a few seconds after the atomization has begun. The ability to observe the atomization process that occurs in a zone below the nozzle tip is lost. Loss of this information is well recognized as being a major impediment to the successful adaption of control devices and strategies to provide process control. It is highly desirable to be able to observe the atomization at the atomization nozzle, and in the atomization zone below the nozzle over an extended period of time to provide process control.
Several important properties of metal powder, and the products formed from consolidation of the powder, are dependent on the as-atomized particle size. These properties include composition homogeneity, mechanical performance, e.g. strength, and toughness, as well as physical characteristics of the powder itself, e.g., particle shape, porosity, and flow qualities. Most of these properties improve as particle size decreases, however, powder handling becomes more complicated for finer powder because of caking, environmental contamination, pyrophorosity and other affects.
The strong dependence of properties on particle size translates into an increased demand for atomization process control that provides a predetermined particle size range, and minimizes the production of powder having a particle size above or below the predetermined range. Such process control can be improved by viewing the interaction of the atomizing gas jet and the liquid metal stream in the atomization zone.
An aspect of this invention is to provide an apparatus for viewing the atomization of liquid metal.